Whenever packed towers of appreciable packed height, say generally speaking, in excess of 20 feet, are involved, it is desirable to arrange the packings in a series of vertical beds. There are many reasons for that,--the most obvious ones being (1) excessive weights generated by some packings when employed in one single bed, (2) deteriorating internal liquid distribution in the packed bed, that must be corrected if a reasonable mass transfer efficiency is to be maintained, and (3) the packed heights may have to be broken up into vertical sections to allow for normal process manipulations, as demanded by feed introduction and liquid and gas or vapor withdrawals.
Whenever such a break-up of a vertical bed into seccessive sections is mandatory, there is required a device generally known as a packing support plate, that supports the packed section in the tower.
Such packing support plates may be of many designs, as in "Norton Packed Tower Intervals" Bulletin TA-80, Copyright 1974, Norton Co. Akron, Ohio 44309. Besides supporting packings, these devices permit also various implementations of gas or vapor referred to as "gaseous medium" introductions into the packings.
Of course, since the packings usually operate in counter-current flow, with gas rising and liquid descending, the supports must also allow a simultaneous discharge of liquid from the base of the packed section. Normally, the conventional packing support plates facilitate such a liquid discharge. However, since the liquid thus discharged from such a vertical section must normally be carefully redistributed to the top of the next tower section, the "liquid distribution capability" of common packing support plates is usually not sufficiently good to produce the best mass transfer of the packed section below, which it thus irrigates.
This being the true state of affairs, in order to implement the best functioning of which the packing is capable, it has been the practice to follow up such packing support plates with subsequent liquid redistribution devices to the packed section below.
This practice is particularly well illustrated by the design shown on page 4.0 of the aforementioned Norton Bulletin T-80, where a packing support plate is shown, followed up by such a separate liquid redistributor.
Whereas this separate arrangement may accomplish the desired purposes, namely supporting the packing and redistributing the liquid to the packed section below, it has the obvious drawbacks of much greater cost and lesser convenience than would be experienced if the two functions of packing support and liquid redistribution could be achieved by one single composite apparatus.